For the bearing of a spindle motor used for such as a disk drive device that records and reproduces information by rotating a disk-shaped recording medium such as a hard disk, various kinds of dynamic pressure bearings are proposed that utilize fluid pressure of lubricating fluid such as oil intervening between a shaft and a sleeve to rotatably support the shaft and sleeve for rotation relative to one another.
A spindle motor including such a dynamic pressure bearing is required to prevent a negative pressure from occurring in the oil with a simplified structure while maintaining desired bearing rigidity as well as reducing thickness of the bearing and reducing its cost.
FIG. 13 is a sectional view of a spindle motor proposed to solve the above-described problem (e.g. Japanese Patent Unexamined Publication No. 2003-88042, referred to as literature 1 hereinafter). In this spindle motor, rotor hub 901, shaft 902, and rotating magnet 903 compose rotor 900. Rotor hub 901 includes substantially disk-shaped flange (top plate) 904 and cylinder-shaped back yoke 905 hanging down from the outer circumferential edge of flange 904. One end of shaft 902 is externally fitted to the central part of flange 904 of rotor hub 901. Further, radial dynamic pressure bearings 906, 907, which generate fluid dynamic pressure in the oil while rotor 900 is rotating, are provided on the inner circumferential surface of sleeve 908 and the outer circumferential surface of shaft 902.
At least one of the top end surface of sleeve 908 or flange 904 of rotor hub 901 is provided with dynamic pressure generating grooves (not shown) to compose thrust bearing 909. Here, the dynamic pressure generating grooves are provided so as to apply the oil with a radially inward pressure while rotor 900 is rotating.
A series of minute gaps is formed between the top end surface of sleeve 908 and the bottom surface of flange 904 of rotor hub 901; between the outer circumferential surface of shaft 902 beyond flange 904 and the inner circumferential surface of sleeve 908; and between the end surface of shaft 902 and the inner surface of seal cap 910. The minute gap retain oil therein continuously, forming a dynamic pressure bearing with what is called a fulfilling structure. The end of shaft 902 forms a bearing that utilizes a pressure substantially balancing with the oil pressure within thrust bearing 909. This prevents a negative pressure from occurring in the oil with a simplified structure while maintaining desired bearing rigidity providing for reduced thickness and cost.
FIG. 14 illustrates an example of another conventional spindle motor (e.g. Japanese Patent Unexamined Publication No. 2004-248344, referred to as literature 2 hereinafter). As shown in FIG. 14, this spindle motor has shaft 923 integrally formed with rotor hub 921. Magnetic body 935 which generates magnetic attractive force between magnetic body 935 and field magnet 922 is provided on a part of base plate 931 where it faces field magnet 922, such that thrust force is developed.
In the above-described literature 1, a slight gap may be formed between the flange of the rotor hub and the fixing portion of the shaft, possibly causing oil filled as a dynamic pressure bearing to be drawn into the gap by capillarity. At this moment, the oil can undesirably leak from the boundary surface between the rotor hub on the central part of the top surface of the rotor and the shaft.
When fitting the rotor hub with the shaft, it is not easy to ensure the runout accuracy of the disk-mounting surface in the axial direction of the shaft. Accordingly, when a disk (not shown) is fitted, the axial component of the runout of the disk surface and its variation undesirably increase.
In the above-described literature 2, processing the shaft is difficult due to the cylinder-shaped back yoke, thereby undesirably deteriorating the processing accuracy. Further, the diameter of the shaft is difficult to measure, thereby preventing easy management of the steps of manufacturing spindle motors.
On the base plate, a magnetic body must be fixed to cause a magnetic attractive force between the magnetic body and a field magnet, at a position facing the field magnet, where in order to slim down the spindle motor, the thickness of the magnetic body needs to be reduced as well. However, with a thin magnetic body, it is difficult to ensure adequate strength, thereby causing a distortion in the circumferential direction of the spindle motor. Consequently, the amount of air gap between the field magnet and the magnetic body cannot be stabilized, and thus the axial runout component undesirably occurs upon the rotation of the rotor hub.
Moreover, the magnetic body fixed on the base plate loses its adhesivity due to factors such as aging and temperature change, eventually causing possible desorption of the magnetic body. If desorption occurs, generation of force in the thrust direction ceases, thereby deteriorating the bearing performance.